It is important that the potatoes are as dry as possible, so that they can absorb more fat, because a significant part of French cooking still contains butter. 



For the same purpose, we can boil or bake potatoes in a lot of salt so that they cannot absorb water. After cooking, steam it and pass it through a sieve to achieve a silky texture. 



Then add cream, milk and preferably cold butter to settle on a better texture, taking into account the emulsion process.